Reducing the spread of HIV among intravenous drug users, and from them through heterosexual and perinatal transmission, is an urgent public health problem. Attaining this goal requires a thorough understanding of the behaviors and social networks through which HIV is transmitted, and of the social and cultural factors that affect risk-taking and deliberate risk reduction. Although much has been discovered, much remains to be understood. Unsolved problems include the reasons why white IV drug users are less likely to be infected than Blacks or Latinos; why and how 75% of New York City new injectors remain uninfected for 5 years or more after beginning to inject; the precise mechanisms by which some peer groups become particularly inclined to reduce risks; and the complex pattern of racial/ethnic variations in risk-taking and risk reduction. Specific aims are to determine (1) how patterns of individual behavior and the characteristics of those with whom these individuals take risks (their "risk networks") are associated with low risk of HIV infection; (2) the kinds of social relationships and peer group cultures that are associated with these lower risk behavioral and network patterns; and (3) to see whether and how knowledge of risk networks, social relationships, and peer group cultures helps us to understand the unsolved problems listed above. In order to resolve these issues, we will need to develop better ways to measure and conceptualize risk networks, social relations, and peer group culture of IV drug users, and thus develop new research techniques for general application in the field. The study will first interview a small sample of street-recruited IV drug users to get daily reports of what they do and who they do it with and from this develop a questionnaire that reflects the variations in risk networks, social relationships, and peer cultures. During the next 21 months, a cross-section of 1300 IV drug users of mixed race/ethnicity, including at least 400 who have been injecting no more than 5 years, will be interviewed with this questionnaire and will undergo testing for hepatitis B and HIV infection. Statistical analysis will focus on how risk behaviors and risk networks explain who is infected (with special attention to how risk networks and behaviors differ with years of injection and across race/ethnic groups, and whether these differences explain differences in who is infected); and on how social relations and peer cultures affect risk behaviors and networks. Substudies will deepen our contextual understanding of risk networks, relationships, and cultures; and measure the proportion of newer injectors who are infected with HIV without having seroconverted.